The origin and evolution of a unisexual hybrid: Poecilia formosa

Abstract

Clonal reproduction in vertebrates can always be traced back to hybridization events as all known unisexual vertebrates are hybrids between recognized species or genetically defined races. Interestingly, clonal vertebrates often also rely on interspecific matings for their reproduction because gynogenesis (sperm-dependent parthenogenesis) and hybridogenesis are common modes of propagation. While in most cases these hybridization events leave no hereditary traces in the offspring, occasionally the genome exclusion mechanism fails and either small parts of male genetic material remain inside the oocyte in the form of microchromosomes, or fusion of the sperm nucleus with the oocyte nucleus leads to polyploid individuals. In this review, we highlight the important role of hybridization for the origin and evolution of a unisexual hybrid: the Amazon molly, Poecilia formosa.

Figure 1

Reproductive modes. Capital letters depict genomes—different letters indicate different species. Numbers (0–9) differentiate individual genomes derived from recombination. In sexual reproduction, both mating partners produce haploid germ cells that are unique due to recombination and result in highly variable offspring. In parthenogenesis, females produce unreduced diploid oocytes that develop into all female offspring that is genetically identical to its mother. In hybridogenesis, the female genome passes through generations unchanged while the male genome is exchanged every generation (hemiclonal). In gynogenesis or sperm-dependent parthenogenesis, females produce unreduced oocytes that develop into all female offspring. They need, however, sperm from a closely related sexual species to trigger the onset of embryonic development. The male genetic material does not contribute to the offspring’ genotypes. Reproduction in gynogenesis is truly clonal.

Figure 2

Forms of paternal introgression. Schematic and examples of resulting karyotypes. While during regular gynogenesis no genetic material from the father is included into the genome, in very rare cases the exclusion mechanism can fail either leaving small pieces of genetic material inside the oocyte (microchromosome indicated by asterisk) or fertilizing the diploid egg leading to triploid individuals.